The present invention relates generally to an imaging apparatus and film components thereof for use in electrostatographic, including digital, apparatuses. The film components herein are useful for many purposes including fixing a toner image to a copy substrate, and the like. More specifically, the present invention relates to film components comprising a high modulus polyimide which, in embodiments, is substantially filled with a conductive filler, preferably a doped metal oxide filler, in order to impart a desired resistivity. In specific embodiments, the conductive filler is an antimony doped tin oxide filler. In another embodiment, the film components comprise a polyimide substrate, and an outer layer provided thereon. In yet another embodiment, the film components comprise a polyimide substrate, an intermediate layer provided thereon, and an outer release layer provided on the intermediate layer. The films of the present invention may be useful as fuser members in xerographic machines, especially color machines.
In a typical electrostatographic reproducing apparatus, a light image of an original to be copied is recorded in the form of an electrostatic latent image upon a photosensitive member and the latent image is subsequently rendered visible by the application of electroscopic thermoplastic resin particles which are commonly referred to as toner. The visible toner image is then in a loose powdered form and can be easily disturbed or destroyed. The toner image is usually fixed or fused upon a support which may be the photosensitive member itself or other support sheet such as plain paper.
The use of thermal energy for fixing toner images onto a support member is well known and methods include providing the application of heat and pressure substantially concurrently by various means, a roll pair maintained in pressure contact, a belt member in pressure contact with a roll, a belt member in pressure contact with a heater, and the like. Heat may be applied by heating one or both of the rolls, plate members, or belt members. With a fixing apparatus using a thin film in pressure contact with a heater, the electric power consumption is small, and the warming-up period is significantly reduced or eliminated.
It is important in the fusing process that minimal or no offset of the toner particles from the support to the fuser member take place during normal operations. Toner particles offset onto the fuser member may subsequently transfer to other parts of the machine or onto the support in subsequent copying cycles, thus increasing the background or interfering with the material being copied there. The referred to "hot offset" occurs when the temperature of the toner is increased to a point where the toner particles liquefy and a splitting of the molten toner takes place during the fusing operation with a portion remaining on the fuser member. The hot offset temperature or degradation of the hot offset temperature is a measure of the release property of the fuser, and accordingly it is desired to provide a fusing surface which has a low surface energy to provide the necessary release. To ensure and maintain good release properties of the fuser, it has become customary to apply release agents to the fuser roll during the fusing operation. Typically, these materials are applied as thin films of, for example, silicone oils to prevent toner offset.
Another important method for reducing hot offset, is to impart antistatic and/or field assisted toner transfer properties to the fuser. However, to control the electrical conductivity of the release layer, the conformability and low surface energy properties of the release layer are often affected.
Attempts at controlling the conductivity of the outer layer of fuser members, particularly fuser belts or films, have been accomplished by, for example, adding conductive fillers such as ionic additives to the surface layer of the fuser member.
U.S. Pat. No. 5,411,779 to Nakajima et al. discloses a composite tubular article for a fusing belt comprising a tubular inner layer of polyimide and fluoroplastic outer layers.
U.S. Pat. No. 5,309,210 to Yamamoto discloses a belt apparatus comprising a base layer polyimide and a fluorine resin outer layer.
U.S. Pat. No. 5,149,941 to Hirabayashi and U.S. Pat. No. 5,196,675 to Suzuki disclose an image fixing apparatus comprising an electrically insulating material base layer and low resistance surface layer insulating member comprised of a polyimide.
U.S. Pat. No. 5,532,056 teaches a fixing belt comprised of a polyimide resin.
Attempts have been made to add electrically conductive additives to polymers in order to partially control the resistivity of the polymers. However, to some extent, there are problems associated with the use of these additives. In particular, undissolved particles frequently bloom or migrate to the surface of the polymer and cause an imperfection in the polymer. This leads to a nonuniform resistivity, which in turn, leads to poor antistatic properties and poor mechanical strength. The ionic additives on the surface may interfere with toner release and affect toner offset. The higher temperatures of the fusing process also increase the mobility of the ionic components and increase depletion rates. Furthermore, bubbles appear in the conductive polymer, some of which can only be seen with the aid of a microscope, others of which are large enough to be observed with the naked eye. These bubbles provide the same kind of difficulty as the undissolved particles in the polymer namely, poor or nonuniform electrical properties and poor mechanical properties.
In addition, the ionic additives themselves are sensitive to changes in temperature, humidity, operating time and applied field. These sensitivities often limit the resistivity range. For example, the resistivity usually decreases by up to two orders of magnitude or more as the humidity increases from 20% to 80% relative humidity. This effect limits the operational or process latitude.
Moreover, ion transfer can also occur in these systems. The transfer of ions will lead to contamination problems, which in turn, can reduce the life of the machine. Ion transfer also increases the resistivity of the polymer member after repetitive use. This can limit the process and operational latitude and eventually the ion-filled polymer component will be unusable.
Use of carbon black as a conductive filler has also been disclosed. Carbon black has been the chosen additive for imparting conductive properties in electrostatographic films. Carbon black is relatively inexpensive and very efficient in that a relatively small percentage can impart a high degree of conductivity. However, the blackness of this material makes it difficult and sometimes impossible to fabricate products with the desired level of conductivity. Further, films filled with carbon black have a tendency to slough and thereby contaminate their surroundings with black, conductive debris. In particular, the carbon black can cause undesirable black marks on the copied or printed substrates. Carbon black particles can also impart other specific adverse effects. Such carbon dispersions are difficult to prepare due to carbon agglomeration, and the resulting layers may deform due to random hard carbon agglomerate formation sites as well as non-uniform electrical properties. This can lead to an adverse change in the conformability of the fuser member, which in turn, can lead to insufficient fusing, poor release properties, hot offset, and contamination of other machine parts.
Generally, carbon additives tend to control the resistivities and provide somewhat stable resistivities upon changes in temperature, relative humidity, running time, and leaching out of contamination to photoconductors. However, the required tolerance in the filler loading to achieve the required range of resistivity has been extremely narrow. This, along with the large "batch to batch" variation, leads to the need for extremely tight resistivity control. In addition, carbon filled polymer surfaces have typically had very poor dielectric strength and sometimes significant resistivity dependence on applied fields. This leads to a compromise in the choice of centerline resistivity due to the variability in the electrical properties, which in turn, ultimately leads to a compromise in performance.
Many doped metal oxides offer significant advantages in both color and transparency when compared to carbon black. They are, however, relatively expensive and usually require higher dosages to achieve comparable levels of conductivity. In addition, dispersion of metal oxides can lead to short comings in surface roughness and particle size.
Therefore, a need remains for conductive fusing films for use in electrostatographic machines, wherein the film possesses the desired resistivity without the drawbacks of lack of transparency of the film which may adversely affect its use in color products, especially color imaging systems. Further, a need remains for a conductive film having conductive fillers which impart the desired resistivity without compromising surface roughness. Also, a need remains for films having improved mechanical properties to maintain film or belt integrity for improved flex life and image registration, improved electrical properties including a resistivity within the range desired for superior performance and a decrease in the occurrence of hot offset. Additionally, a need exists for controlling electrostatic transfer functions by neutralizing toner charges, improving chemical stability to liquid developer or toner additives, improving thermal stability for fusing operations, improving comformability, and providing low surface energy and higher modulus. Moreover, a need exists for a film in which the resistivity is uniform and is relatively unaffected by changes in environmental conditions such as changes in humidity, temperature, electrical surges, and the like. These and other needs are achievable with embodiments of the present invention.